The present invention relates to a process for applying electrode layers to a polymer electrolyte membrane strip in a desired pattern.
Fuel cells convert a fuel and an oxidizing agent, physically separated from one another, into electricity, heat and water at two electrodes. Hydrogen or a hydrogen-rich gas can be used as the fuel and oxygen or air as the oxidizing agent. The energy conversion process in the fuel cell is distinguished by particularly high efficiency. For this reason, fuel cells in combination with electric motors are gaining increasing importance as an alternative to conventional combustion engines for automotive vehicles.
The so-called polymer electrolyte membrane fuel cell (PEMFC) is suitable for use as an energy converter in motor vehicles thanks to its compact construction, its power density and its high efficiency.
The central component in a PEMFC is the so-called membrane-electrode assembly (MEA). This consists of a polymer electrolyte membrane which is provided on both sides with a catalytically active layer. One of the layers takes the form of an anode for the oxidation of hydrogen and the second layer takes the form of a cathode for the reduction of oxygen. In order to bring the gaseous reaction media (hydrogen and air) to the catalytically active layers and, at the same time, to establish an electrical contact, so-called gas diffusers or gas diffusion structures are placed on the anode and cathode layers. These are usually carbon fiber paper or nonwoven carbon fabric which allow the reaction gases good access to the electrodes and permit good discharge of the cell current. Single PEMFCs or PEMFC stacks are built up from these membrane-electrode assemblies.
The polymer electrolyte membrane consists of proton conducting polymer materials. These materials are also referred to below as ionomers for short. Tetrafluoroethylene-flourovinyl ether copolymer with sulfonic acid groups is preferably used. This material is marketed for example by E. I. du Pont with the trade name NAFION(copyright). However, other, especially flourine-free, ionomer materials such as sulfonated polyether ketones or aryl ketones or polybenzimidazoles may also be used. For use in fuel cells, these membranes generally have a thickness of between 10 and 200 um.
The anode and cathode contain so-called electrocatalysts, which cataytically support the respective reaction (oxidation of hydrogen or reduction of oxygen). The metals of the platinum group of the periodic table are preferably used as the catalytically active components. For the most part, so-called supported catalysts are used, in which the catalytically active platinum group metals have been applied, in fine-particle form, to the surface of a conductive support material. The average crystallite size of the platinum group metal is between about 1 and 10 nm. Fine particle-size carbon blacks have proved suitable as support materials.
Many different processes exist for the production of membrane-electrode assemblies, but only a few of them are suitable for industrial purposes, i.e. for continuous manufacture on a large scale at economic costs.
In DE 195 09 749 (corresponding to U.S. Pat. No. 5,761,793) a process for the continuous production of a composite of electrode material, catalyst material and a solid electrolyte membrane is described, wherein a catalytic layer of a catalytic powder comprising the electrode material, the catalyst material and the solid electrolyte material is produced on a support. This catalytic layer is heated on a side facing away from the support to soften the solid electrolyte material and is applied on to the solid electrolyte membrane by rolling under pressure. The entire disclosure of U.S. Pat. No. 5,761,793 is incorporated herein by reference.
The polymer electrolyte membrane is coated over this entire surface using this process. The application of the electrodes in a two-dimensional pattern, also referred to below as selective coating, as is absolutely essential for the production of fuel cell stacks, can only be achieved with great effort. For this purpose, appropriately shaped separators which prevent the catalyst material from being bonded to the polymer electrolyte membrane are placed on the membrane. The areas of the support overlapping the separators can then be removed with a knife after the support has been applied on to the membrane by rolling.
In addition, this process is associated with a high risk of pinhole formation in the membrane, since the material for the electrode layers is used in powder form. If coarser powder particles are present in the catalytic material, the thermally softened membrane is perforated when the powder is rolled under high pressure.
In WO 97/50142 (CA 128: 104377 F/WPI 1998-077409) a continuous process for coating a polymer electrolyte membrane with electrodes is described, in which a polymer membrane strip is drawn through a bath of platinum salt solution. The adhering salt is then reduced to the precious metal in a gas stream or in another bath. With this process again, it is only possible to coat the polymer electrolyte membrane over its entire surface. The membrane can be damaged by the strongly acidic metal salt solutions and the reducing bath.
Another process is described in DE 195 48 421 (CA 127: 138102 G/WPI 1997-351291) for the production of a membrane-electrode assembly wherein the bonding of the polymer electrolyte membrane, the electrode layers and the gas diffusion layer is carried out continuously in a rolling process. The polymer membrane is coated with the electrode layers over its entire surface in this process.
Accordingly, it is an object of the present invention to provide a continuous and inexpensive process for coating a polymer electrolyte membrane strip with electrode layers in a desired pattern, which also allows the required pattern to be produced simply and with high dimensional stability. In addition, the risk of pinhole formation in the membrane by the coating process should largely be avoided.
The above and other objects of the invention can be achieved by a process for applying electrode layers on to a polymer electrolyte membrane strip in a desired pattern, wherein the front and back of the membrane are continuously printed with the electrode layers in the desired pattern using an ink containing an electrocatalyst and the printed electrode layers are dried at elevated temperature immediately after the printing operation, the printing taking place while maintaining accurate positioning of the patterns of the electrode layers on the front and back in relation to one another.
For use in the process according to the invention the starting material is a polymer electrolyte membrane strip supplied, for example, in a roll. The process is carried out on a strip-coating line with various stations. This line is equipped with a minimum of an unwinding device, a printing station, a drying station and a winding device, and several deflector and guide rolls as required.
The strip speed can vary within relatively broad limits and its upper limit is set only by the constraints of the printing process selected. After leaving the printing station, the polymer strip with the still fresh electrode layers passes through the drying station. The electrode layers are therefore dried at elevated temperature immediately after the printing operation. A continuous circulating air drier may be used as the drying station, for example. Infrared driers are also suitable. The preferred temperatures for drying the layers are between 60 and 150xc2x0 C. The residence time of the polymer membrane in the drying station must guarantee adequate drying of the electrode layers. It depends on the temperature selected and can be prolonged by appropriate deflections in the drying station.
The printing process takes place with an ink which contains an electrocatalyst. This ink is often also referred to as a paste because of its consistency. In addition to a high boiling-point solvent it contains, for example, one or more electrocatalysts, proton-conducting ionomer and optionally auxiliaries such as wetting agents, pore forming agents or similar. To produce these inks, the components are processed to an intimate mixture with the aid of high shear forces. This guarantees good dispersion of the electrocatalyst in the solvent. Inks and pastes suitable for the process according to the invention are described in DE 19611510 A1 (U.S. Pat. No. 5,861,222), P 19810485.5 (as yet unpublished German patent application) and P 19837669.3 (as yet unpublished German patent application).
Various printing processes may be used for printing the membrane, such as for example screen printing, offset, gravure or pad printing. Depending on the printing process used and the desired layer thickness of the electrode layers, the consistency of the ink must be adjusted accordingly, in order to obtain optimum results. The necessary measures are known to the expert. The layer thicknesses for the electrodes are in the range of between 5 and 100, preferably between 5 and 15 um. If the necessary layer thickness is not achieved with one printing operation, the membrane may be printed more than once.